In the field of automotive vehicles, it is an urgent need to address global environmental problems such as regulations on CO2 emission. On the other hand, in terms of ensuring passenger safety, collision safety standards of automotive vehicles have been reinforced and a structure design capable of sufficiently ensuring safety in a boarding space is in progress. To simultaneously achieve these requests, it is effective to use a high-strength steel sheet having a tensile strength of 780 MPa or more as a structure member of an automotive vehicle and reduce the weight of a vehicle body by further thinning this high-strength steel sheet. However, since processability is deteriorated if the strength of a steel sheet is increased, an improvement of processability is an unavoidable problem in applying the above high-strength steel sheet to an automotive member.
TRIP (Transformation Induced Plasticity) steel sheets are known as steel sheets having both strength and processability. As one type of TRIP steel sheets, TBF (TRIP aided bainitic ferrite) steel sheets whose parent phase is bainitic ferrite and which contain retained austenite (hereinafter, written as “retained γ” in some cases) are known, for example, as disclosed in patent literatures 1 to 4. In TBF steel sheets, high strength is obtained by hard bainitic ferrite and good elongation (EL) and stretch flange formability (λ) are obtained by fine retained γ present on boundaries of bainitic ferrite.
In addition to the above properties, an improvement of low-temperature toughness is desired for a collision safety improvement at low temperatures. However, TRIP steel sheets are known to be inferior in low-temperature toughness and low-temperature toughness has not been considered at all thus far.